The invention pertains to the field of light projection systems employing a deformable transparent light modulating medium which diffracts incident light to form projected images in accordance with the modulating information. More particularly, the invention relates to such systems in which the light modulating medium is a fluid overlying a rotating disk and information is written by an electron beam which scans a raster on the fluid and forms orthogonally related diffraction gratings which project superimposed information necessary for color projection, systems of this type having been commonly termed light valve projection systems. The diffraction gratings are composed of one set of gratings parallel to the scanning lines which contain green color information, and two sets of diffraction gratings orthogonal to said scanning lines containing red and blue color information.
Whereas light valve projection systems have been in commercial use for many years and are capable of providing high grade performance, numerous problems have existed with respect to these systems of differing degrees of importance. A number of these problems such as directed to the composition and processing of the fluid medium and overall light efficiency of the projected image have received substantially satisfactory solution. However, other difficulties have continued to plague workers in their efforts to further perfect the systems. One has been termed "burn-in" or after glow effects which result in a moire pattern of color, predominantly green, appearing within the projected picture ostensibly due to elemental portions of the diffraction gratings remaining for an inordinate length of time. Although this effect may not be readily visible in a full color image projection, tending to blend into the various colors of the image, it may become highly visible in alphanumeric displays and other forms of dark field projections.
Another problem includes nonuniformity in the fluid thickness within the raster which gives rise to nonuniformity in the efficiency of light transmission across the raster. One specific manifestation of this is a thickening of the fluid across one corner of the raster which is evidenced by a line appearing across a corresponding corner of the projected picture, termed "tailback". Another manifestation is a thinning of the fluid in a region at the bottom of the raster which results in reduced light transmission efficiency or picture brightness and, together with excess substrate deposit that also tends to occur in this region, may ultimately result in a curved line across a corresponding region of the picture. This phenomenon is called "necklace" effect.
In addition there is a condition known as "red field flutter" which is manifested by a flashing of light intensity principally in the red field. This effect is believed to result primarily from partial cancellation of the extremely fine red diffraction gratings in response to a slightly discontinuous movement in disk rotation.